1. Field of the Invention
The present invention concerns a method of covering shadow areas in a cellular mobile radio system and a radio booster for implementing this method.
2. Description of the Prior Art
In cellular mobile radio systems the environment of fixed and mobile transceiver stations includes obstacles due to the terrain or to buildings, for example.
Because radio propagation is more or less directional, radio shadow areas exist in the vicinity of these obstacles, i.e. areas in which radio transmissions can be partially blocked or absorbed by some feature of the environment, with the result that they can be strongly attenuated.
Shadow areas can also exist at the extreme range of the transmitters of the fixed stations of the network.
Accordingly, the signal transmitted can deteriorate significantly.
Various solutions to this problem have been put forward.
A first solution is to use a radio booster comprising a transceiver and an associated antenna provided with an amplifier located in the shadow area. The radio receiver is a broadband receiver, i.e. it can receive and retransmit a major part of the frequency spectrum used in the radio network to which it belongs.
Its function is to receive radio transmissions in the shadow area, to amplify them and then to retransmit them on the same frequency, at a different angle. It is therefore virtually "transparent", i.e. it modifies only the amplitude of the retransmitted signal. The advantage of using a booster is that it is autonomous: no cables have to be connected to the booster. This solution is therefore of low cost.
This solution is unsatisfactory, however, because of a phenomenon similar to the Larsen effect that occurs, especially if the booster is not far away from the fixed transceiver station it relays. The Larsen effect is a spurious oscillation which occurs when the output of an electro-acoustic system, such as the sound from a loudspeaker, for example, impinges on its input, usually the microphone, and the effect produced is a whistling. In the case of radio waves this phenomenon produces interference in the signal transmitted.
One solution to this problem is to use a booster which converts or translates the frequency of the signals received before retransmitting them. A solution of this kind is described in patent application GB-A-2 253 324, for example.
It has been found that this solution is unsatisfactory in the case of cellular mobile radio systems because the base transceiver station cannot determine if frequency translation has been applied unless devices to carry out the converse frequency translation are used at the boundaries between cells, which is costly in hardware. Another solution for cellular mobile radio systems avoids these problems. This solution increases the number of cells, and therefore the number of base transceiver stations, by creating additional cells to cover the shadow areas.
This solution is effective and has the advantage of avoiding the problems that arise with the previous solution.
It has drawbacks of its own, however.
First of all, it is very costly in installation terms because, unlike a radio booster, a cellular network base transceiver station must be connected by cables to a base station controller and to a network operation and maintenance center.
Also, given its complexity, a cellular network base transceiver station is inherently a high cost item.
An object of the present invention is therefore to provide a method of covering shadow areas in a cellular mobile radio system that is not costly to implement but which guarantees good quality radio signals so that the base transceiver stations can determine if the signals received are at a frequency resulting from frequency translation in a booster without needing to use devices to carry out the converse frequency translation.